In 1955, representatives of the Polish Army approached the Main Institute of Aviation in Warsaw with a proposal to develop a small observation helicopter of simple construction and easy to maintain. They based their request on information about similar achievements in the United States and Western Europe. The subject was very interesting and, as it soon turned out, very difficult to accomplish.
The new helicopter was designed in 1955-56 by a team of engineers led by Jerzy Kotlinski. The basis of the design was the adoption of a single-rotor configuration with direct drive of the main rotor by ramjet engines. The fuselage structure was made of welded steel tubes, and the landing gear consisted of two shock-absorbing duralumin tube skids. On a vertical column, the main gearbox and the head of the two-blade main rotor with two control paddles (the so-called Hiller head) were mounted. In the rear section of the tail boom, a tail rotor was installed, driven by a drive shaft from the main gearbox. In the forward section of the fuselage, ahead of the rotor column, the pilot’s seat, control devices, and an instrument panel were installed.
Behind the pilot was a 100-litre fuel tank, sufficient for 15 minutes of engine operation. On the rotor head, a shut-off valve and a fuel pump were mounted, which transferred fuel to the engines through lines running inside the rotor blades. The ramjet engines, with a thrust of 12.5 kG (at a forward speed of 180 m/s), were fuelled by petrol. They were developed by engineer Stanislaw Wojcicki, who had previously worked on this form of propulsion. A technological curiosity was the all-metal main rotor blades filled with polyvinyl chloride (PVC) foam.
In 1957, ground testing of the helicopter began on a test stand, initially without and then with the engine running. Serious problems then emerged related to rotor blade vibrations (so-called flutter). Attempts to fully eliminate them were unsuccessful. On 21 June 1957, during one of the ground experiments, a tragic accident occurred in which test pilot engineer Antoni Smigiel was killed. The accident was caused by an engine that broke free from the rotor blade. The helicopter was completely destroyed. After a thorough analysis of the event and the introduction of modifications to the fuselage and powerplant design, ground tests were resumed on the second prototype. To improve test safety, remote control was employed. In October 1958, another accident occurred, caused by a failure of an operating engine. After this incident, testing was permanently discontinued.
The final decision to terminate trials was influenced by their results to date and an analysis of the practical usefulness of such a flying machine. A comparison with the results of similar experiments abroad was not encouraging. Virtually all ground and flight experiments with “jet-powered” helicopters had ended in failure. The most serious problem was the impossibility of safe landing in the event of a powerplant failure, due to the high rate of descent in autorotation. The ramjet engines proved to be inefficient and too noisy under the operating conditions at the tips of the short blades. They caused significant additional centrifugal loading on the rotor system, which necessitated providing the hub and rotor with correspondingly high structural strength, which in turn forced an increase in the weight of these components.
The “Trzmiel” (Bumblebee) never made any flight; it was an interesting but impractical technical achievement. The last prototype of the helicopter was donated to the museum collection.
| Rotor diameter | 7.0 m |
| Fuselage length | 2.95 m |
| Takeoff weight | 340 kg |
| Maximum speed | 131 km/h |
| Ceiling | 5500 m |
| Range | 15 minutes of flight |
| Engine | 2 x ramjet engines with 12.5 kG thrust at 180 m/s |
